kal is just the kryptonian clark. they're the same person, just different halves. clark is the simpler farm boy, and kal is the more rigid and computer-like kryptonian.
kal only really came about inbetween seasons 3 and 4 when clark was sucked into a phantom-zone-like part of krypton. there, jor-el (or jor-el's spirit) apparently got his fatherly chance to raise and/or develop kal-el, as he would have, given the chance on krypton.
whether or not the actual message from jor-el was to "dominate the earth" is up for grabs and open to interpretation.
its really just showing the differences between clark's human and alien lives.
